1. Field of the Invention
The present invention relates to an optical multilayer structure having a function of reflecting, transmitting, or absorbing incident light, an optical switching device using the same, and an image display using the same.
2. Description of the Related Art
In recent years, importance of a display as a display device of video information is increasing. As a device for the display and, further, as a device for optical communication, an optical memory device, an optical printer, and the like, development of an optical switching device (light valve) operating at high speed is in demand. Conventionally, as devices of this kind, there are a device using a liquid crystal, a device using a micro mirror (digital micro mirror device (DMD), trade mark of Texas Instruments), a device using a diffraction grating (Grating Light Valve (GLV™) of Silicon Light Machines (SLM)), and the like.
A GLV is obtained by fabricating a diffraction grating with an MEMS (Micro Electro Mechanical Systems) structure, and a high speed light switching device of 10 ns is realized with an electrostatic force. The DMD performs switching of light by moving a mirror in the MEMS structure. Although a display such as a projector can be realized by using any of the devices, since the operation speed of each a liquid crystal and the DMD is slow, to realize a display as a light valve, the devices have to be arranged two-dimensionally, so that the structure becomes complicated. On the other hand, the GLV is of a high speed driving type. By scanning a one-dimensional array, a projection display can be realized.
However, the GLV has a diffraction grating structure and is therefore complicated, for example, since six devices have to be formed per pixel and diffracted rays emitted in two directions have to be converged to one by an optical system.
Light valves each of which can be realized with simple configuration are disclosed in U.S. Pat. Nos. 5,589,974 and 5,500,761. The light valve has a structure in which a translucent thin film having a refractive index of √ns is provided on a substrate (having a refractive index of ns) via a gap portion (gap layer). In this device, the thin film is driven by using an electrostatic force to change the distance between the substrate and the thin film, that is, the size of the gap portion, thereby transmitting or reflecting a light signal. The refractive index of the thin film is √ns in contrast to the refractive index ns of the substrate. It is said that, by satisfying such a relation, light modulation of high contrast can be carried out.
A device having the configuration as described above, however, has a problem such that, when the refractive index ns of the substrate is not a large value like “4”, the light valve cannot be realized in a visible light region. From the viewpoint of the structure, desirably, the translucent thin film is made of, a material such as silicon nitride (Si3N4) (having refractive index n=2.0). The refractive index ns of the substrate in this case is equal to four. In the visible light region, it is difficult to obtain such a transparent substrate and a choice of options of materials is narrow. At a wavelength for communication of infrared rays, the light valve can be realized by using germanium (Ge) (n=4). It seems difficult to apply the material for a use of a display in reality.